The present invention pertains to the separation and recovery of fluorine compounds from multicomponent fluids containing fluorine compounds, and in particular to the purification of nitrogen trifluoride (NF.sub.3) by continuous cryogenic distillation.
Fluorine compounds are widely used in the manufacture of semiconductors. These compounds can be costly and detrimental to the environment, if released. Nitrogen trifluoride is used as an etching gas and a chamber cleaning gas.
In the process of manufacturing nitrogen trifluoride a gas mixture is created that contains nitrogen trifluoride, nitrogen, oxygen, nitrous oxide, hydrogen fluoride and other components. Nitrogen and oxygen are more volatile than nitrogen trifluoride, while nitrous oxide and hydrogen fluoride are less volatile and may form solids at low temperature and higher concentrations. This gas mixture may be then purified by cryogenic distillation to obtain nitrogen trifluoride after removal of bulk levels of active fluorides and heavy components, if high purity nitrogen trifluoride is required.
Batch distillation is a cost-effective method of purifying relatively small quantities of product, because it is simple and offers considerable operating flexibility. This economy is lost when large quantities of product must be purified. This is because batch distillation requires storage for the feed and intermediate product between the production batches. Storage of large amounts of gases is troublesome. It can be accomplished either by compression into storage vessels or by liquefaction. Both methods are expensive due to the capital cost of large storage vessels and the operating costs associated with frequent batch transfers.
Batch processing also is a source of environmentally detrimental releases of fluorine compounds during frequent transfers, process line purges, and storage vessel ventings.
Cryogenic distillation of fluorine compounds with the addition of a wash liquid is disclosed in U.S. Pat. Nos. 5,502,969 (Jin, et al.), 5,626,023 (Fisher, et al.) and 5,771,713 (Fisher). The wash liquid is added to absorb fluorine compounds from a carrier gas or to prevent some components from solidifying. One (or two or three) Cryogenic distillation column(s) is used to separate highly volatile fluorine compounds from the wash liquid. (Alternatively, the subsequent columns separate less volatile fluorine compounds either from wash liquid or from residual heavy components.) All of these distillation systems involve batch processing.
A continuous process for ultra-high purity nitrogen trifluoride production is disclosed in U.S. Pat. No. 5,832,746 (Nagamura). The process comprises pressurizing a nitrogen trifluoride feed gas; eliminating moisture, CO.sub.2 and partially CF.sub.4 from the feed gas; cooling the feed gas down and passing it through a cold adsorber to eliminate N.sub.2 F.sub.2, N.sub.2 F.sub.4, N.sub.2 O and further reduce CF.sub.4 content; liquefying the feed gas in a bottom reboiler/condenser of a medium pressure distillation column; and using the supplied heat to separate the heavy components of the feed gas in that column. The resulting mixture is then passed to a low pressure distillation column, heat integrated with the medium pressure distillation column by means of a second reboiler/condenser, where it is distilled to provide ultra-high purity NF.sub.3 as a bottom product and a waste vapor overhead. This process could be relatively expensive because of the need for pressurization of the NF.sub.3 feed gas. The process also could be difficult to control because of the double heat integration (between the feed gas and the medium pressure distillation column and between the medium and the low pressure distillation columns).
In all of these prior art distillation processes, reflux is provided by condensing a portion of an overhead vapor against cryogenic liquid, for example liquid nitrogen. This requires the use of a condenser and associated control instruments.
Other methods of purification of NF.sub.3 feed gas from certain components are disclosed in the patent literature. In U.S. Pat. No. 5,069,887 (Suenaga, et al.) an adsorption process on a molecular sieve is used to separate NF.sub.3 from CF.sub.4. In U.S. Pat. No. 5,183,647 (Harada, et al.) conditions that favor chemical decomposition of N.sub.2 F.sub.2 from NF.sub.3 feed gas are disclosed. U.S. Pat. No. 5,779,863 (Ha, et al.) discloses a method for separating and purifying perfluorocompounds (PFC's), including NF.sub.3, using a distillation column system comprising three or four distillation columns.
It is desired to have a safe and economical, continuous cryogenic distillation process for purification of nitrogen trifluoride.
It is further desired to have such a process which accomplishes the desired separation with maximum recovery, without any releases of fluorine compounds to the environment.
It is still further desired to have a process for recovering nitrogen trifluoride from multicomponent fluids which overcomes the difficulties and disadvantages of the prior art to provide better and more advantageous results.